CP6100 I/O Process Programming Manual

Introduction
between controllers. In a single-port controller configuration, |
each controller must be associated with a different processor. |
The exact division of work depends on the traffic you expect on
the lines, and on the overhead for controlling each line; simply
assigning half the lines to one controller and half the lines to
the other might not provide the best possible balance.
Another important feature accounts for the flexibility and
performance of the subsystem: the interface to each line (called
a "line interface unit," or LIU) has its microcode loaded from a
disk file. Because it is programmable, the same hardware can run
asynchronous, byte synchronous, or bit synchronous protocols, and
can handle many elements of protocol formerly handled by I/O
processes. (With the earlier controllers, this off-loading of
work from the I/O process would have entailed a separate
microcode set for each protocol.) By reducing the work of I/O
processes, and hence of the processors in which they run, the
6100 subsystem may improve the throughput not only on individual
lines, but also in the NonStop System as a whole.
To enhance the flexibility of each LIU, hardware modules for
different electrical interfaces are available for the subsystem.
To connect a line that supports a specific electrical interface
(e.g., RS-232, RS-449), you fit the LIU with a matching interface
module.
A final advantage of the 6100 subsystem is the potential for
connecting more lines per controller than was possible before.
Whereas a byte or bit synchronous controller could support only
four lines, the new subsystem can support up to fifteen lines per
controller pair.
October 1985
1-11